The present invention pertains to deposition of solder paste on printed wiring boards and more particularly to a method and an apparatus for uniform deposition of the solder paste on the printed wiring board.
Various coatings may be applied to a substrate by application of the coating through a stencil to the substrate. For example, in the fabrication of a printed wiring board (PWB) for the placement of surface mounted components, solder, pads are deposited on the PWB. A stencil which has apertures in the shape of the solder pads to be applied to the printed wiring board is placed in intimate contact to the printed wiring board and held firmly together with the PWB. An amount of solder paste which forms the solder pads is applied to the top surface of the stencil. A squeegee is then drawn through the solder paste and over the entire stencil and the solder paste is pushed into each of the apertures or forms of the stencil to form the solder pads on the PWB.
Ideally, the solder paste is to be deposited evenly across the entire length of each aperture of the stencil. As the squeegee blade draws the solder paste over an aperture of the stencil, the solder paste is deposited from the leading edge of the aperture at a lower level to the trailing edge of the aperture at a higher level where the amount of paste deposited at the trailing edge is much greater than the amount of paste deposited at the leading edge. Further, the paste is deposited in a non-linear fashion between the two levels at the leading and trailing edges of the aperture.
Attempts have been made to repeat the above process drawing the squeegee blade through the solder paste in the opposite direction over the stencil aperture. The results obtained have been that the solder paste is high at both the leading and trailing edges of the stencil aperture, but low in the middle of the aperture. As a result, the solder paste is not uniform and components mounted in such an arrangement may shift or move or completely dislodge with subsequent printed wiring board processing. In addition for the situation in which a second squeegee pass is made, the time required for the deposition process has been doubled.
It would be highly desirable to provide an arrangement for applying solder paste evenly to a printed wiring board through an aperture of a stencil.